Configure strict testing via phpunit.xml.

This `strict="true"` option sets the `reportUselessTests`,
`strictCoverage`, `disallowTestOutput`, `enforceTimeLimit`, and
`disallowTodoAnnotatedTests` options when run through the configuration
parser.  It will also add other new strict options as phpunit upstream
adds them.
Merge pull request #33 from nubs/master

Configure strict testing via phpunit.xml.